German Published Patent Application 23 39 896 shows a circuit including an element having a specific breakdown voltage coupled to the control electrode of the switching transistor in the primary circuit between the primary winding and the contact-break distance of the ignition transistor. If the voltage exceeds a permissible value when the contact-break distance of the ignition transistor makes the transition to the nonconducting state, then the voltage at the element having a specific breakdown voltage breaks through, and a control current begins to flow across the control path of the ignition transistor, which control current again makes the emitter-collector path of the ignition transistor somewhat permeable to current. As a result, the voltage at the contact-break distance of the ignition transistor drops again, and, in fact, continues to drop until the voltage at the switching element having a specific breakdown voltage falls below this breakdown voltage. This configuration comprising the element having a fixed breakdown voltage (Zener diode) cannot ensure overvoltage protection for all operating ranges. For example, if one designs the ignition system and, thus, the element having a specific breakdown voltage, so as to allow an adequate secondary voltage to still be provided for all operating states, given a large secondary load, then larger values can occur on the parts energized with high voltage, given a low secondary load. Such an overloading can lead to their destruction, when, for example, a spark plug connector drops out and a breakdown occurs across the high-voltage insulation.
As such, conventional systems work with a fixed primary Zener-type characteristic as a voltage bracketing of the ignition transistor and, thus, do not provide a satisfactory compromise between a sufficient secondary voltage supply and high-voltage strength of the parts that are energized with high voltage.